
Here \(N_2(e^{\sqrt{-1}{\theta }},b)\)  is trivial if \((b_2-b_3)\sin {\theta }>0\) , or non-trivial
if \((b_2-b_3)\sin {\theta }<0\) , in the sense of Definition 1.8.11 on p.41 of [1]}. Note that
by Theorem 1.5.1 on pp.24-25 and (1.4.7)-(1.4.8) on p.18 of [1]}, when \({\lambda }=-1\)  there hold
\(c_2 \ne  0\)  if and only if \(\dim \ker (M_2(-1,c)+I)=1\)  and
\(c_2 = 0\)  if and only if \(\dim \ker (M_2(-1,c)+I)=2\) .
For more details, readers may refer Section 1.4-1.8 of [1]}.
